Our prior studies have affirmed an important functional role of the carotid body (CB) chemoreflex in the pathophysiology of chronic heart failure (CHF). CB chemoreflex sensitivity is markedly increased and contributes to increased sympathetic nerve activity (SNA) and breathing irregularities in CHF. Selective carotid body denervation (CBD) stabilizes control of breathing, reduces SNA, and improves cardiac autonomic and baroreflex function in CHF rats and rabbits. We hypothesize that two important factors contribute to the exaggerated CB chemoreflex and its impact on sympathetic and ventilatory function in CHF. First, a chronic reduction in CB blood flow secondary to LV dysfunction reduces expression of an the endothelial shear-stress transcription factor Kruppel-like factor 2 (KLF2), an important activator of antioxidant and anti-inflammatory pathways, precipitating an enhanced CB input to the central nervous system to induce progressive breathing instability and elevated SNA. In Aim 1, we propose that flow-mediated KLF2 down-regulation causes CB- mediated sympatho-respiratory dysfunction in CHF. These experiments will compare KLF2 expression and its functional sympatho-respiratory effects in the CB and within central regions of the chemoreflex (commNTS, PVN, RVLM) between low flow (myocardial infarct, MI) and high flow (A-V fistula, AV) models of CHF in rats. Second, we propose that CB hypo-perfusion and development of irregular breathing together lead to ischemic- intermittent hypoxia (IH) and activation of HIF-1?. In Aim 2, we propose that activation of HIF-1? further contributes to CB-mediated sympatho-respiratory dysfunction in CHF. These experiments will compare HIF1-?/HIF2-? expression in the CB and commNTS, PVN, RVLM between MI- and AV-CHF models and the functional effects of altered HIF1-? expression in these regions. Our prior studies also have shown that CBD reduces aberrant ventricular remodeling and arrhythmias and progressive deterioration of left ventricular function with an outcome of improved survival in CHF. Thus, chronic activation of the peripheral chemoreflex pathway contributes to the progression of cardiac dysfunction in CHF. To explain improved cardiac function in CHF by CBD, we hypothesize (Aim 3) that the CB-mediated reflex SNA activation in CHF exacerbates impaired renal function and blood volume (BV) homeostasis and, importantly, induces splanchnic venoconstriction to translocate BV to the thorax and elevate cardiac preload. These experiments will assess the impact of CB chemoreflex activation and of CBD on renal function, BV, venous capacitance, and cardiac preload in CHF rabbits. Lastly, the beneficial effects of exercise training (ExT) in CHF may be related to improved CB chemoreflex function. We will assess the effects of ExT in each of the aims through potential mechanisms KLF2 up-regulation (Aim 1), HIF-1? down-regulation (Aim 2), and normalized CB reflex control of renal function, BV and splanchnic venoconstriction to reduce cardiac preload (Aim 3).